P0420 indicates the computer thinks the catalytic converter isn't working as well as it should. This is determined by the PCM evaluating the switch rate of the front and rear o2 sensors. Once the rear o2 sensor switch rate reaches a point close to the front o2 sensor it sets the code. This can be a result of degenerated catalytic converter, or in some rare instances, bad o2 sensors. In most cases repairing this condition requires replacing the catalytic converter with an OEM unit. Aftermarket converters don't have the same quality and quantity of the precious metals that make up the substrate found inside. This usually results in the code setting again in just a few months.

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TT
- Faulty Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1
- Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit fuse
- Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit open shorted to ground
- Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit poor electrical connection
- Faulty Engine Control Module (ECM)
Usually the code indicates a bad Oxygen sensor, but an intake vacuum leak or a failing fuel pump can cause the code, so check your intake vacuum hose for a leak or hole. But most likely, you have a faulty Oxygen sensor. Now Bank 1 Sensor 1 means the bad O2 sensor is the one on the number one cylinder side (which is the cylinder closest to the timing belt or where all your belts and pulleys are),which is bank one. If you look really closely, you will see that the number 1 cylinder is just a little bit closer to the belts than the number 2 cylinder. Now that you know which side you are dealing with, the next step is which sensor. There are two O2 sensors on each side (upstream or downstream), four total. Sensor 1 is the upstream sensor, which is before the catalytic converter closest to the exhaust manifold. Number 2 sensor is on or after the catalytic converter. So in short, you are replacing the O2 sensor on the Number 1 cylinder side before the catalytic converter. They are a pain to remove. Get some Pb Blaster lubricating oil and spray on the threads and let sit for 10 minutes and spray it again. You will need a 7/8 Oxygen sensor removal socket to remove it. Don't forget to disconnect negative battery terminal and O2 wire harness first.. Any questions please feel free to email me. Good luck.

More than likely the catalytic converter has failed. the computer looks at the downstream O2 sensor to determine if the cat is working. If it sees the downstream O2 starting to switch back and forth like the upstream O2 or read consistently higher than it should than it will flag the P0420 code.

Hi, P0420 code means a catalytic converter fault.
The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.

Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).